MTU with Site to Site VPN

New to this so bear with me. Just set up a site to site IPSec connection with a site we have in Brasil. On the US side is a Cisco VPN 3005 concentrator and in Brasil is a Netgear VPN/Router. I'm seeing alot of dropped packets in the Cisco stats so I'm guessing we might have a MTU issue? They are on a ADSL connection, we are on a T1...both sites set at 1500 MTU. When I do a ping -f -l from the US to their router, I can go up to 1472. When I do it through the VPN, I can go up to 1272. Does this mean the MTU should be set to 1300? And if so, is that just on their router or do I have to set the concentrator to the same MTU considering we have US based people who VPN into it using the cisco client?

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The Other Mike
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